1. Field of the Invention
The invention concerns a method of controlling the angular mechanical speed of a load driven in rotation by an electromechanical drive train with little damping, and possibly having non-linearities such as transmission play, in the absence of a measurement of angular speed of the load.
The invention also concerns a device for implementing this speed control method.
Throughout the text, the term “load” designates indiscriminately any mechanism that can be driven in rotation by means of an electromagnetic drive train with little damping. Thus, without being limited thereto, the load can be composed of a roller of a rolling mill or paper manufacturing machine, a ship propeller, a railroad drive axle, etc.
Moreover, the expression “electromagnetic drive train” designates an assembly including a torque actuator formed by a power converter and an electric motor of any type, as well as a drive train through which this actuator drives the load.
Finally, the expression “with little damping” means that the electromechanical drive train has mechanical resonance frequencies. These frequencies are defined by the drive train's parameters, such as inertia and rigidity. In the context of the invention, these parameters are not well known or vary during operation, which results in uncertainty or variation of the resonance frequencies of the electromagnetic drive train and of the associated amplitudes.
2. Description of Related Art
Diagramatically represented in FIG. 1 of the attached drawings is a known type of closed loop servo system providing the control of the speed of rotation of a load 1, driven by a torque actuator 2 through a drive train 3. The torque actuator 2 includes an electric motor 4 coupled to a power converter 5.
The control of the speed of the motor 4 is provided by a controller 6 that receives a reference signal Ωm* representative of the desired speed of the motor 4. When there is no measurement of the speed of the load 1, the controller 6 also receives a signal Ωm representative of the actual speed of the motor 4. In response, the controller 6 issues a torque reference signal Γ*, which controls the torque actuator 2.
Among existing controllers, for example, are PI (Proportional Integral), PID (Proportional Integral Derivative), H∞, QFT (Quantitative Feedback Theory), and RST.
In the frequency domain, and using Laplace transforms, it is known that the response of a controller is characterized by a rational transfer function C(s), which corresponds to the ratio of two polynomials N(s)/D(s), where s designates the complex operational variable of the system in question. The transfer function of the open loop system that includes the controller is represented in the Black, Nyquist, or Bode plan, by a curve called “frequential pattern,” which shows the changes in gain and phase difference between the system's input and output, as a function of the real part ω of the operational variable s.
The different controllers are distinguished from each other particularly by the degree of each of the polynomials N(s) and D(s), that is, by their number of zeros (roots of the numerator) and poles (roots of the denominator). When the system is high order, with little damping, that is, when it has a high number of poles and zeros, only a high order controller makes it possible to meet the objectives of robustness in performance and stability. However, this also results in increasing the number of coefficients of the controller's polynomials, and this consequently makes the optimal adjustment of the controller more difficult.
When a PI or PID controller is used, the degrees of the associated polynomials are two and three coefficients, respectively. The adjustment is therefore simple. However, to achieve the objectives of robustness in performance and stability is no longer possible in the presence of an electromechanical drive train characterized by resonant modes, and possibly by non-linearities such as transmission play.
To remedy the difficulties that occur when a PI or PID controller is used in an electromechanical drive train with little damping, it has been proposed to associate with said controller a state feedback based on a signal issued by an estimator. This solution is described in the thesis of Marius Goslar entitled “Ein Beitrag zur anwendungsorientierten Zustands-regelung elektrischer Hochleistungsantriebe,” presented on Aug. 14, 1998 and published in Great Britain by “Conservatree Print & Design” ISBN 0953473503.
More precisely, the abovementioned thesis concerns a case in which the angular speed of the load is known. It is proposed to use the genetic algorithms to optimize the coefficients of the polynomials of the controller.
This known solution provides good damping of the system when resonance frequencies are present. However, it is still not very satisfactory in terms of robustness. Moreover, the use of the genetic algorithms to calculate the coefficients of the polynomials of the controller makes the adjustment of said controller particularly complex.
When an H∞, QFT, or RST controller is used, the degrees of the polynomials are appreciably higher. These controllers are called “robust” because they allow the controlled system to preserve its stability and performance under nominal conditions, that is, where there are uncertainties about the parameters, and possibly nonlinearities in the transmission, such as transmission play. However, they have a high number of coefficients that makes their adjustment particularly complex.
Regardless of the type of controller used, the robust control of the angular speed of the load is made difficult when it is not possible to have a direct measurement of this speed.